1. Field of the Invention
The present invention relates to a switching power supply apparatus, and specifically to a switching power supply apparatus provided with an overcurrent protection function.
2. Description of the Related Art
A switching power supply apparatus includes a power conversion circuit that converts an input voltage input from an input power supply into an output voltage supplied to a load, a switch device that switches the input voltage, a switching control circuit that controls the switch device, and the like. The switching control circuit is provided with an overcurrent protection function of protecting the switch device, a diode, an inductor, and the like against problems such as overheating or damage of components due to, for example, an overcurrent. For example, Japanese Unexamined Patent Application Publication No. 2008-206271 discloses a switching power apparatus configured to limit input/output power by turning off a switch device when it is detected that a current flowing through the switch device has reached a predetermined threshold, thereby performing an overcurrent protection operation.
In a switching power supply apparatus such as the one disclosed in Japanese Unexamined Patent Application Publication No. 2008-206271, when it is detected that a current (trapezoidal-wave current) flowing through a switch device has reached a predetermined threshold, the switch device is turned off at that point of time. As a result, the peak value of the current flowing through the switch device is limited, whereby overcurrent protection is realized. In other words, since switching is immediately stopped when the current flowing through the switch device has exceeded the threshold, the peak value of the current flowing through the switch device is limited to an overcurrent corresponding to the threshold.
However, there may be a case in which defects of devices such as a switch device, a diode, and an inductor are not caused by an overcurrent itself but caused by overheating due to the overcurrent. Hence, if control is performed in such a manner that the switching is stopped only on the basis of the determination result of comparison between a current flowing through a switch device and a threshold, as in Japanese Unexamined Patent Application Publication No. 2008-206271, the following problems may be encountered.
For example, an overcurrent protection operation that is performed in such a manner that a switching operation is immediately stopped when a load current, which continuously flows in a steady state, gradually increases to a certain threshold (overcurrent) is rational in terms of its objective. However, in the case where a switching power supply apparatus with a rating of 100 W for heat generation intermittently drives a 200 W load with a duty cycle of 50%, when a threshold is set at 110 W output, overcurrent protection is performed right after a 200 W load has been started to be driven and, hence, driving for a desired operation cannot be performed at all. When a 200 W load is driven with a duty cycle of 50%, the average power is 100 W and, hence, no heat problems are generated for the devices such as a switch device, a diode, and an inductor of the switching power supply apparatus, and no defects due to heat are generated. In other words, the apparatus can be designed in such a manner that no defects are generated. If the switching power supply apparatus is designed, from the beginning, in such a manner as to have a power rating of 200 W, the average power is more than that necessary for a load with an average of 100 W. In other words, this results in overdesign in terms of size and cost.
Further, when control is performed in such a manner that switching is immediately stopped only on the basis of the determination result of comparison between a current flowing through a switch device and a threshold, as in the switching power supply apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2008-206271, in the case where the waveform of a current flowing through the switch device is a half-wave sine wave, the switch device is tuned off at the peak of the current. Hence, this may cause a problem in that the current waveform suddenly changes from a half-wave sine wave to a quarter-wave sine wave and the resonance condition is no longer satisfied, causing significant distortion of an operation waveform, resulting in an increase in the size of an output voltage ripple, generation of audible noise, and generation of defective operations in the switching power supply apparatus.